1. Field of the Invention
The present invention relates to a method of producing a hot pressed product or component, and more particularly relates to the method of producing a hot pressed product or component of complex shape wherein the material from which the component is formed is hot pressed between a plurality of die parts.
2. Description of the Prior Art
Manufacture or production of hot pressed products or components or complex shapes has heretofore posed a number of serious problems in the art of powder metallurgy. Since, as has been said in this art, powders do not follow the laws of hydrodynamics and do not flow around corners, and since pressures can economically be applied only from the top and bottom of conventional die apparatus, a number of product design limitations have resulted. For example, such shape limitations as undercuts and reverse tapers could not previously be molded, nor could such complex components as turbine blades, helical gears, etc. For some contours it has often been necessary to leave certain areas for subsequent machining or other finishing operations. While many of the typical limitations have been overcome, nevertheless the complicated tooling that has been required has rendered such means uneconomical.
Under the usual conditions of conventional hot pressing, it has not been possible heretofore to produce a sintered component having a complex shape and a uniform density. For example, when conventional tools have been used during hot pressing, to produce complex shapes, the problem has arisen that the shape of the component changes as the size of the component decreases, whereas the shape of the tools have necessarily remained constant during hot pressing. This wording applies to hot pressing a preform and while it is true what is said, it would be normal practice in such circumstances to use a powder charge which leads to non-uniform compaction ratio, causing non-uniformity of density etc. This has tended to lead to distortion of the component and to a final product which has a non-uniform density and therefore internal lines of weakness.
Presently used for the production of components of complex shapes is the so-called isostatic hot pressing method. According to this method, the powdered material from which the component is made is compacted uniformly in all directions in a preformed flexible mold that is immersed in fluid in a pressure vessel. While this technique has led to the obtainment of components of more uniform density and virtually eliminated die wall friction, it has nevertheless has a number of serious drawbacks, especially in the case of sintering of powders having a high melting point such as the borides, nitrides, carbides, etc. In such a case the major problem is the containment of the material to be hot pressed in a die without explosion of the die band. Moreover, since during isostatic hot pressing, the powder is sintered in a closed container, the moisture and gases contained therein are released and consequently react with the powder itself. When the powder is nitride powder, decomposition of the nitride is promoted under these conditions. By means of the present invention, however, these known disadvantages can be overcome, and the production of components of complex shapes and uniform densities can be cheaply and conveniently effected with conventional apparatus and equipment. Using the method of this invention, it is possible to ensure that the tools conform exactly, or with a minor discrepancy that can be taken up during hot pressing, to the shape of the component throughout the hot pressing process.